#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define ss second
#define ff first
#define pb push_back
#define mp make_pair


int main(){
    ios_base::sync_with_stdio(0);
    string x;
    cin>>x;
    x+="@";
    int s=0;
    for(int i=1;i<x.size()-1;i++){
        if(x[i-1]=='1'&&x[i+1]=='1')x[i]='!';
    }
    for(int i=1;i<x.size();i++){
        if(x[i]=='1'||x[i]=='!')s++;
    }
    cout<<s<<endl;
    return 0;
}